该章主要介绍网络安全技术。包括网络安全基本内容、网络的安全威胁、网络攻击和防御方法、常用的加密技术、防火墙技术、网络欺骗技术。目的是希望同学能够了解网络安全的基本知识，掌握网络维护的基本内容和方法。重点难点：网络安全涉及的内容；网络安全的威胁；网络攻击的常用方法；网络防御的方法；常用的加密技术；常用的防火墙技术；常用的网络欺骗技术

Administrative matters Instructors: Eytan Modiano Meeting times: Tuesdays and Thursdays Text: Communications Systems engineering Proakis and salehi Grading 10% weekly Homework Assignments 30% each of 3 exams Final exam during final exam period

Sampling Given a continuous time waveform, can we represent it using discrete samples? How often should we sample? Can we reproduce the original waveform?

Source coding Source Encode Channel Alphabet alphabet Source symbols Letters of alphabet, ASClI symbols, English

Channel Coding When transmitting over a noisy channel, some of the bits are received with errors Example: Binary Symmetric Channel (BSc) Pe= probability oferror Q: How can these errors be removed? A: Coding: the addition of redundant bits that help us determine what was sent with greater accuracy

Modulation Representing digital signals as analog waveforms Baseband signals Signals whose frequency components are concentrated around zero Passband signals Signals whose frequency components are centered at some frequency fc away from zero Baseband signals can be converted to passband signals through modulation Multiplication by a sinusoid with frequency fc Eytan Modiano

Used for analyzing network performance In packet networks, events are random Random packet arrivals Random packet lengths While at the physical layer we were concerned with bit-error-rate, at the network layer we care about delays How long does a packet spend waiting in buffers?

Routing Must choose routes for various origin destination pairs o/d pairs) or for various sessions Datagram routing: route chosen on a packet by packet basis Using datagram routing is an easy way to split paths Virtual circuit routing: route chosen a session by session basis Static routing: route chosen in a prearranged way based on O/D pairs Eytan Modiano

Packet Multiple Access SHARED UPLIN TERMINAL APPL TERMINALL TRANS NET LLC DLC MAC

Communication Defined The transmission of information and understanding through the use of common symbols from one person to another Common symbols may be